1. Field of the Invention
The present invention relates to a sheet stacking apparatus and an image forming apparatus configured to stack a large number of sheets on a sheet stacking portion. More particularly, the present invention relates to a sheet stacking apparatus and an image forming apparatus configured to stack sheets discharged at high speed from a main body of the image forming apparatus, with precise alignment.
2. Description of the Related Art
In recent years, thanks to technological advances, an image forming apparatus has become capable of forming images at higher speed. Together with the increase in image forming speed, sheet discharging speed from the image forming apparatus has also increased. As a result, demand for a high-volume sheet stacking apparatus with precise alignment capability is increasing.
Japanese Patent Application Laid-Open No. 2006-124052, for example, discusses a sheet stacking apparatus which includes a pressing member that presses a sheet against a sheet rack so that the sheet can be discharged to the sheet rack more speedily.
FIG. 18 illustrates a configuration of a conventional sheet stacking apparatus which enables high-volume output. The sheet stacking apparatus is attached to a conveying belt 508 that rotates clockwise and includes a gripper 503. The gripper 503 rotates together with the conveying belt 508 to convey a sheet while holding a leading edge of the sheet. Further, the sheet stacking apparatus includes a leading edge pressing member 506 and a trailing edge pressing member 507 configured to press down a leading edge and a trailing edge of a sheet.
In the sheet stacking apparatus having such a configuration, a sheet discharged from an image forming apparatus (not shown) is received by an inlet roller 501 and then a leading edge of the sheet is turned over to the gripper 503 by a conveyance roller 502. Then, the conveying belt 508 rotates, and the gripper 503 moves together with the conveying belt 508 while holding the leading edge of the sheet. In this way, the sheet is conveyed along the upper portion of the sheet stacking portion 505.
When the leading edge of the sheet abuts a leading edge stopper 504, the gripper 503 releases the sheet so that the sheet is discharged onto the sheet stacking portion 505. In this manner, a predetermined number of sheets are stacked. Every time a sheet is stacked, an alignment member (not shown) performs a jogging process in a direction perpendicular to the sheet conveying direction (hereinafter referred to as width direction) so that alignment of the sheets is improved.
When sheets are stacked at high speed, the possibility of a sheet jam, occurring when a sheet interferes with a trailing edge of a preceding sheet stacked on the sheet stacking portion 505, is increased. Therefore, during sheet stacking, the leading edge pressing member 506 and the trailing edge pressing member 507 press down a leading edge and a trailing edge of a sheet so that the sheet is quickly discharged to the sheet stacking portion 505.
In other words, when sheets are stacked at high speed, the leading edge pressing member 506 and the trailing edge pressing member 507 press a leading edge and a trailing edge of a sheet against the sheet stacking portion 505 at the time the sheet is discharged to the sheet stacking portion 505 so that the sheet is out of the way of the next sheet.
However, in such a conventional sheet stacking apparatus and an image forming apparatus having such an sheet stacking apparatus, a size of the sheet stacking portion 505 is determined according to a maximum size of a sheet to be stacked. Further, only a single stack of sheets is allowed in the sheet stacking portion 505. Accordingly, even if a sheet which is half the size of the maximum-size sheet is stacked, the number of sheets that can be stacked is the same as the number of maximum-size sheets. Accordingly, an unused space X shown in FIG. 18 appears in the sheet stacking portion 505.
In other words, in the conventional sheet stacking apparatus, even when a sheet-stackable space exists in the sheet stacking portion 505, the space is not used for the purpose of stacking sheets. Therefore, there has been a problem that a sheet-stackable space in the sheet stacking portion 505 is not effectively used.
In order to solve this problem, Japanese Patent Application Laid-Open No. 9-255213, for example, discusses an apparatus which is capable of stacking two stacks of sheets. This apparatus enables stacking of two stacks of half-size sheets (for example, A4 landscape) on a sheet stacking portion which is configured to stack a maximum length of a sheet (for example, A3 portrait).
However, since this apparatus utilizes space by stacking two stacks of half-size sheets on a sheet stacking portion by changing sheet discharging positions, no adequate margin of space is left on the sheet stacking portion. Thus, when a sheet is discharged beyond its stacking space, it affects its adjacent stacking space. In particular, when a sheet is stacked starting from an upstream stacking space, the sheet tends to go beyond its stacking space to the downstream stacking space by a discharging force and a case of misalignment can increase. Further, it is possible that a stack of sheets leans on the other stack, or a stack pushes the other stack in the sheet discharging direction. Consequently, stacking capacity of the apparatus decreases.